Answer 4.8

a) Explain the basic principle of ion cyclotron resonance (ICR).

An ion of velocity v entering a uniform magnetic field B perpendicular to its direction will move on a circular path by action of the Lorentz force. The cyclotron angular frequency is independent of the ion’s initial velocity, but a function of its mass, charge, and the magnetic field. By applying a transverse electric field alternating at the cyclotron frequency the ions are accelerated. As the ions accelerate, the radius of their orbit increases, and the resulting overall motion is a spiral. For lighter ions, the spiral reaches the same radius with fewer cycles than in case of heavier ones, i.e., the spiral is steeper, because low-mass ions need less energy than high-mass ions to accelerate to a certain velocity.

b) What is FT-ICR?

Fourier transform ion cyclotron resonance.

c) What are the differences in ion detection between ICR and FT-ICR?

The working principle of the first-generation ICR instruments essentially was to perform an energy scan: the m/z value was obtained from the number of half cycles, until the ions struck an electrometer plate.The disadvantages of this concept are: i) mass accuracy and resolution are limited to 1/Nc (Nc = number of half cycles); ii) the electric signal for ion detection is solely due to neutralization of the ions; and iii) the ions are removed from the cell upon detection.
FT-ICR circumvents the above disadvantages of scanning ICR instruments. Detection is based on the measurement of image cur-rents in the detector plates, i.e., detection in FT-ICR means “listening to the circulating ions”. The transient free induction decay (FID) is recorded, and afterwards, the FID is converted from the time domain to the frequency domain by means of Fourier transformation. The frequencies are converted to m/z values; their amplitudes now representing the abundances of the corresponding ions.

d) Name major advantages of the mode of ion detection employed in FT-ICR.

Advantages of FT-ICR-MS are: i) the 1/Nc limit vanishes; ii) sensitivity improves because the ions give rise to a detectable image charge during each passage of a detector plate; and iii) ion detection is non-destructive, i.e., ions are not lost upon detection giving the opportunity to perform MS/MS experiments.

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